Addition polymers (in contrast to condensation polymers) can be depolymerized by heat to simpler monomers and its oligomers. Use of one or more catalysts can result in lower reaction temperatures at which the depolymerization reaction occurs, as well as provide some amount of control on the depolymerized product mixture. However, the product mix will always contain large quantities of unsaturated compounds that will affect its stability on exposure to air. Also, without further purification steps such as one or more fractionations, the depolymerized reaction product cannot be used directly.
Various plastics are examples of compounds produced by addition polymerization reactions. Typically, such plastics are produced from non-renewable petroleum resources and are often non-biodegradable. In the United States, such plastics like polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC) and polystyrene (PS) are produced in amounts exceeding 115,000 million pounds annually. Plastics are used in many industries to form products for sale in both industrial and residential markets. In industrial markets, these polymers are used to form packaging, insulation, construction products, and the like. In residential markets, these polymers are used to form bottles, containers, and the like.
Catalytic depolymerization of high and low density polyethylene, polypropylene and polystyrene into diesel like fuel is well known in the literature, both published and patented. Usability of such fuel is hampered by the fact that the presence of large amounts of unsaturated products reduces the stability (formation of brown polymer products) of the diesel fuel produced thereby and, as such, necessitates the need for a separate hydrogenation step to improve the stability and calorific value of the diesel so produced.
Accordingly, the depolymerization of polymers requires a careful selection of catalysts, processing as well as separation scheme to extract valuable diesel like fuel. FIG. 1 presents a gas chromatograph illustrating the identity and relative quantities of various monomers formed from pyrolysis of polyethylene and polypropylene. The monomers include alkanes, alkenes, and alkynes or dienes having from 2 to 40 carbon atoms wherein the alkanes are colored green, the alkenes are colored red, and the alkynes and dienes are colored blue.
Besides the addition polymers, there are condensation polymers, which include polyesters (PET), polyurethanes (PU), nylons or polyimides and the like. There are also thermoset polymers (example automotive coatings), which are three dimensional polymer networks formed by cross-linking reactions of the linear polymers.
In contrast to polyethylene, polypropylene and other polyenes, condensation polymers and thermosets cannot be “depolymerized” using thermal energy. Instead, one must rely on extensive chemical reactions to convert such products back to their starting materials and, as such, this is economically prohibitive to perform.
Given the above, only a small fraction of the polymers produced are recycled and re-used. Polymers that are not recycled and re-used present potential environmental pollution risks when discarded, are not utilized for energy or raw materials, and contribute to an increased reliance on non-renewable petroleum resources.